The mitochondrial system of animals is an excellent one in which to study molecular evolution. Its DNA is easily purified, contains genes for 13 proteins, 2 ribosomal RNAs and 22 transfer RNAs, yet is small enough to analyze completely. Mitochondrial DNA (mtDNA) lacks intervening, spacer and repetitive sequences, making its analysis uncomplicated. The same genes are present in all animals, but the gene arrangement can change. MtDNA is inherited maternally and is thus free of complications caused by biparental inheritance. The sequence of vertebrate mtDNA changes rapidly, relative to nuclear DNA, due to both base substitutions and deletion/additions. The most extensive changes occur in one small but interesting region (the D-loop region) which contains the regulatory elements for replication and gene expression. Comparisons in this region will allow us to identify sequences that may be important to these functions. Changes in vertebrate mtDNA can be correlated with time, via the fossil record. Two basic questions regarding the evolution of vertebrate mtDNA will be addressed: (1) What kinds of sequence changes occur and in what proportions, and (2) how can a high rate of sequence change be tolerated in the genome of an organelle whose functions are so vital for the cell? To answer these, mtDNA will be obtained from a series of species whose divergence times are well known. The same mitochondrial genes will be cloned and sequenced from each. The sequences will be compared, and the kinds and relative proportions of the changes determined. Changes will be analyzed with respect to the functions of the genes in which they occur. Useful information about constraints on molecular structure and function and how constraints are affected by genome size should be obtained. Also, two instances of sequence rearrangement in the D-loop will be analyzed in order to gain insight into the mechanistic basis of rearrangements. A long-term study of mitochondrial DNA from other major animal groups will be initiated. The immediate goals of the study are to define the total range of variation present in animal mtDNA and to test the generality of the knowledge that has been obtained from studies of vertebrate mtDNAs. The groups chosen include, respectively, the causative agents of schistosomiasis and their intermediate hosts. Knowledge of the biology of these organisms may ultimately lead to methods of control.